Energy is the capacity to do work. In physics, that’s the formal definition: if something can exert a force and move an object, it has energy. To do 100 joules of work, you need 100 joules of energy. But the word “energy” stretches far beyond physics class. It describes the fuel in your food, the power running through your home’s wiring, the reactions inside your cells, and even the subjective feeling of being alert and motivated. Each of these uses connects back to the same core idea, though the details look very different.
Energy in Physics: The Capacity for Work
At its most fundamental, energy is what lets things happen. A rolling ball has energy because it can push something when it hits it. A compressed spring has energy because it can launch a projectile when released. A hot stove has energy because it can heat a pot of water. In every case, energy is the ability to apply force over a distance, which physicists call “work.”
One of the most important principles in all of science is that energy cannot be created or destroyed. It can only change form. This is the first law of thermodynamics, sometimes called the law of conservation of energy. When you drop a ball from a height, the energy stored in its position (gravitational energy) converts into the energy of motion (kinetic energy) as it falls. The total amount stays the same. This principle holds true everywhere, from car engines to stars.
There’s a catch, though. Every time energy changes form, some of it becomes heat that spreads out and becomes harder to use. That’s the second law of thermodynamics in plain terms. You never lose energy, but you lose the ability to do useful things with all of it. A car engine converts chemical energy from gasoline into motion, but a large portion escapes as waste heat through the exhaust and radiator.
Forms Energy Takes
All energy falls into two broad categories: potential energy (stored and waiting) and kinetic energy (actively in motion).
Potential energy includes:
- Chemical energy: stored in the bonds between atoms. Gasoline, food, batteries, and wood all contain chemical energy that gets released during reactions like combustion or digestion.
- Gravitational energy: stored in an object’s height. The higher and heavier something is, the more energy it holds. Hydroelectric dams convert this stored energy into electricity by letting water fall through turbines.
- Nuclear energy: stored in the core of atoms. Splitting heavy atoms apart or fusing light atoms together releases enormous amounts of energy, far more per unit of mass than any chemical reaction.
- Elastic energy: stored in stretched or compressed objects like rubber bands and springs.
Kinetic energy includes:
- Motion energy: any moving object carries it. A car crash is a dramatic example, where all the motion energy releases in an uncontrolled instant.
- Thermal energy (heat): the energy of atoms and molecules vibrating and bouncing around within a substance. The faster they move, the hotter the material.
- Radiant energy: electromagnetic waves, including visible light, radio waves, X-rays, and the sunshine that makes life on Earth possible.
- Sound energy: pressure waves moving through a substance. It tends to carry less energy than other forms.
- Electrical energy: the movement of charged particles, usually electrons flowing through a wire. Lightning is a natural, uncontrolled version of this.
How Energy Is Measured
The standard scientific unit of energy is the joule. One joule equals the energy needed to push with a force of one newton over a distance of one meter. For everyday purposes, though, different fields use different units because joules can be inconveniently small or large depending on the context.
Your electric bill uses kilowatt-hours. One kilowatt-hour equals 3.6 million joules, roughly the energy needed to run a microwave for an hour. Food labels use calories (technically kilocalories), where one calorie equals about 4,184 joules. Heating and cooling systems in some countries use British Thermal Units (BTUs), with one BTU equaling about 1,055 joules. These are all measuring the same thing, just at different scales for different purposes.
Energy in Your Body
Your body is an energy conversion machine. You take in chemical energy from food, break it down, and use it to power everything from your heartbeat to your thoughts. The three macronutrients supply different amounts: carbohydrates provide 4 calories per gram, protein also provides 4 calories per gram, and fat provides 9 calories per gram. This is why fatty foods are so calorie-dense.
At the cellular level, your body converts food into a molecule that acts as a universal energy currency. This molecule stores energy in the bonds between its phosphate groups, and when a bond breaks, the cell captures that released energy to fuel muscle contractions, nerve signals, chemical reactions, and thousands of other processes. Your cells constantly build and break down this molecule, recycling it over and over throughout the day.
Your basal metabolic rate, the energy your body burns just to keep you alive at rest, accounts for the largest share of the calories you use each day. Breathing, circulating blood, maintaining body temperature, and repairing cells all require a constant energy supply even while you sleep. Physical activity and digestion add to that baseline, but for most people, simply existing burns the majority of their daily calories.
Mass and Energy: Einstein’s Famous Equation
In 1905, Einstein showed that mass and energy are two expressions of the same thing. His equation, E = mc², says that the energy contained in matter equals its mass multiplied by the speed of light squared. Because the speed of light is about 300 million meters per second, squaring it produces an astronomically large number. That means even a tiny amount of mass holds a staggering amount of energy.
This isn’t just theoretical. Nuclear power plants and nuclear weapons both convert small amounts of mass into energy. When heavy atoms split in a reactor, the fragments weigh slightly less than the original atom. That “missing” mass has become energy. Einstein himself called this relationship the most important result of his special theory of relativity, and over a century later it continues to hold up under experimental testing.
The “Energy” You Feel
When people say “I have no energy today,” they’re using the word in a way that overlaps with physics but isn’t quite the same thing. Physical energy is precisely measurable in joules or calories. The feeling of being energetic, what researchers call psychological energy, is something different. It’s inferred from your motivation, mood, and behavior rather than measured with an instrument.
Physical energy is purely about magnitude: how many joules, how many calories. Psychological energy has qualities that physics-based energy doesn’t. It has direction (you feel driven toward something), inner quality (it can feel positive or negative), and it fluctuates with sleep, stress, illness, and emotional state. Vitality, the subjective sense of vigor and wellness, blends both physical and psychological energy. You might have plenty of calories available in your body but still feel drained because of poor sleep or low mood.
This distinction matters because boosting one type doesn’t automatically fix the other. Eating more food increases your caloric energy supply, but it won’t necessarily make you feel more energetic if the underlying issue is stress, depression, or disrupted sleep. Conversely, a burst of excitement or motivation can make you feel full of energy even when you haven’t eaten recently.